Pretreating process for phosphatetreating steel sheets and plated steel sheets



Unit

ABSTRACT OF THE DISCLOSURE The present invention provides for a methodof phosphatizing the surface of steel sheets or plated steel sheets,wherein said method is preceded by a pretreating process,

the improvement according to which the pretreating process comprisesapplying to the surface of said steel sheets, a liquid suspension of aninsoluble phosphate selected from the group consisting of zincphosphate, calcium phosphate, magnesium phosphate, ferrous phosphate,ferric phosphate, and aluminum phosphate.

This invention relates generally to pretreating processes forphosphate-treating the surface of ferrous metals and more particularlyto economical pretreating processes for depositing phosphate films onsteel sheets or plated steel sheets very quickly and compactly.

The present invention is a pretreating process to be carried out beforephosphate films are applied to steel sheets or plated steel sheets suchas galvanized steel sheets and comprises treating the surfaces of suchsteel sheets or plated steel sheets with a suspension of an insolublephosphate of bivalent or trivalent metal. If such pretreatment isapplied, in the subsequent phosphate treatment, a phosphate film will beproduced within a very short time and will be very fine and compact.

Generally, in depositing a phosphate film on the surface of a steelsheet or plated steel sheet, the pretreatment of the surface is soimportant that, if no pretreatment is applied at all, no film will beproduced at all or only a phosphate film of coarse crystals will beobtained in the subsequent phosphate treatment. It seems to besubstantially ineffective to pretreatment to use a solution prepared bydissolving such soluble phosphate as, for example, Zn(H PO Currently,the prior art has adopted a pretreating liquid comprising a solution ofsoluble sodium phosphate containing titanium. However, as a result ofinvestigations we have made, it has been disclosed that the ingredientcontributing to the formation of a phosphate film of fine crystals isthe titanium in the solution. Further, with the decrease of titanium, wehave found that the pretreating solution will be aged, that is, thecoating weight of phosphate will increase and the crystal grains of thephosphate film will become coarse. As a result of the aging process thesoluble phosphate in such pretreating liquid is not effective.

In order to obtain a suitable phosphate film by the above describedprior art methods, the pretreating liquid must be frequently replaced sothat a fixed amount of titanium may be always present. Thus thisconventional pretreating liquid has decided disadvantages in that it istroublesome and costly.

Further, with such pretreating liquid, as described above, in order toform very fine phosphate film crystals on a steel sheet or a platedsteel sheet, the concentration States Patent 0 3,395,5Z Patented July30, 1968 of the liquid must be high and therefore the unit price of theproduct is much higher than that of the present invention. Even if theconcentration of the liquid is kept high and the steel sheet is treatedin said pretreating liquid and then further in a phosphate-treatingliquid, the crystal size of the phosphate film will be almost invariablyabove 3 and rarely below 1 1..

The prior art also utilizes a mechanical pretreating process wherein nopretreating liquid is used therein. According to this process, thesurface of a steel sheet is rubbed with a proper rubbing tool before itis dipped in a phosphate-treating liquid. However, in such mechanicalprocess, in order to carefully uniformly rub the surface of the steelsheet, a device of a considerably high precision is required. Therefore,it can not be recommended.

The present invention is designed to eliminate the defects of the priorart processes as described above. As a result of our research for a veryeconomical and efficient pretreating process, we have now invented a newpretreating process for phosphate treatment.

According to the present invention, a suspension of an insolublephosphate of a bivalent or trivalent metal is used so that the surfaceof a steel sheet may be mechanically rubbed and ground, and fine crystalnuclei of the phosphate may be then imparted to the surface thereof.Thus by the present invention, fine crystals of the phosphate may beproduced uniformly on the surface of the steel sheet within a very shorttime.

An object of the present invention is to provide a pretreating processfor the phosphate treatment of metal surfaces wherein a favorable finecompact phosphate film can be formed within a very short time on thesurface of a steel sheet or a plated steel sheet such as a galvanizedsteel sheet.

Another object of the present invention is to provide a pretreatingliquid which is economical and easy to apply by spraying or dipping in apretreating process for phosphate-treating steel sheets or plated steelsheets.

Other objects and a fuller understanding of the invention may be had byreferring to the following description and claims.

The present invention shall be described in detail in the following.

In the pretreating process according to the present invention, there isused an insoluble phosphate of a bivalent or trivalent metal suspendedin water. For example, the insoluble phosphate of the bivalent metal iszinc phosphate, calcium phosphate, magnesium phosphate or ferrousphosphate and that of the trivalent metal is ferric phosphate oraluminum phosphate. In the case of a low concentration (up to about 3g./liter), if the suspension is sprayed onto the surface of a steelsheet or a plated steel sheet under a high pressure, a favorableuniforrr pretreated surface will be obtained. In order to obtain afavorable result with a liquid of a high concentration, usually thesteel sheet or plated steel sheet is dipped in the liquid or, after thedipping, the deposited liquid is pressed or rubbed onto the surface ofthe sheet with a roll or the like.

In the above mentioned spraying method under a high pressure, thefavorable concentration of the suspension is more than 1 g./liter.However, if there is no restriction to the spraying manner, the higherthe concentration, the better the result. Specifically a concentrationof more than 5 g./liter is preferable. A proper concentration may beselected in response to the performances of the nozzle.

A spraying pressure of more than 0.5 kg./cm. is preferable. The higherthe spraying pressure, the better the result. Also, the longer thespraying time, the better the result. But a spraying time of less than 1minute is sulficient.

The spraying pressure, the spraying time and the concentration of thesuspension are so closely correlated with one another that, when thespraying pressure is restricted, and the spraying time is made longer, afavorable result will be obtained and, when the spraying time isrestricted, and the concentration of the suspension is elevated, afavorable result will also be obtained.

The above mentioned high. pressure spraying method is effective to asuspension of a low concentration. But, when the concentration of thesuspension is high, said method will be ditticult to apply in view ofthe equipment. In the case of a suspension of such high concentration,only dipping the steel sheet in the suspension will be effective.Further repeating the dripping or squeezing with a roll after thedipping is elfective.

The preferable pH of the suspension is slightly acid or alkaline atabout 3 to 10. Outside this range, the steel sheet will corrode or willdissolve in the phosphate with an unfavorable result. It is proper toadjust the pH with phosphoric acid or sodium phosphate.

The bivalent or trivalent metal which is the principal agent in thepresent pretreating process may be used as crushed from crystals or asprepared from a soluble salt of the bivalent or trivalent metal or byadding a phosphate to the carbonate or oxide of such metal.

In the present invention, the higher the treating ternperature, thebetter the result. But the effect of the temperature is rather small.Thus the treatment may be applied at to 90 C.

When no pretreatment is applied at all, the crystal size of the obtainedphosphate film will be so coarse as to be more than 100w but, when thepresent pretreatment is applied, the crystal size will be so fine as tobe 2a or less. The time required to complete the phosphate treatmentwill be generally less than 2 minutes when the conventional pretreatingprocess is carried out but will be so short as to be specifically lessthan 10 seconds when the present pretreating process is carried out.

Examples of the present invention are as follows:

Example 1 A fabricated steel sheet was dipped in an alkali solution soas to be degreased, was washed with water and was sprayed under thefollowing conditions:

Ferric phosphate g./liters Liquid temperature C.-- pH 9.8 Sprayingpressure kg./cm. 3.0 Spraying time minutes 1 After the sprayingtreatment, the fabricated steel sheet was washed with running water andwas then treated with phosphate-treatin g bath.

The time required to complete the phosphate treatment was only 10seconds when the pretreatment of the present invention had been appliedbut was more than 5 minutes, sometimes no film being produced, when nopretreatment had been applied at all. The crystal size of the producedphosphate film was so fine as to be 2 when the present pretreatment hadbeen applied.

Example 2 An electrogalvanized steel sheet was sprayed under thefollowing conditions:

Zinc phosphate 'g./liters 25 Liquid temperature C. 30 pH 7.8 Sprayingpressure Kg./cm. 2.5 Spraying time seconds 5 After the sprayingtreatment, the sheet was squeezed with a roll and was soon treated withphosphate-treating bath.

When the present pretreatment had been applied, the time required tocomplete the phosphate treatment was about 10 seconds and the crystalsize of the produced phosphate film was so fine as to be l t.

Example 3 A hot-dipped galvanized steel sheet was dipped under thefollowing conditions:

After the dipping treatment, the sheet was squeezed with a rubber roll,was washed with water and was treated with phosphate-treating bath.

When the present pretreatment had been applied, the time required tocomplete the phosphate treatment was about 15 seconds and the crystalsize of the produced phos phate film was so fine as to be 2,4.

While the present invention has been specifically described herein Withreference to the preferred embodiments of the invention, it is to beunderstood that the present invention may be otherwise practiced than asspecifically described within the scope and spirit of the appendedclaims.

What we claim is:

1. In a method of phosphatizing the surface of steel sheets or platedsteel sheets, wherein said method is preceded by a pretreating process,the improvement according to which the pretreating process comprisesapplying to the surface of said steel sheets, a liquid aqueoussuspension of an insoluble phosphate selected from the group consistingof zinc phosphate, calcium phosphate, magnesium phosphate, ferrousphosphate, ferric phosphate, and aluminum phosphate, in which saidliquid suspension contains more than 1 g./l. of said phosphate andwherein the pH of the liquid is adjusted to from 3 to 10.

2. A process according to claim 1 wherein the suspension is sprayed ontothe surface of the steel sheet under a spraying pressure of more than0.5 l g./cm.

3. A process according to claim 2 wherein the temperature of the liquidsuspension is 10 to C.

4. A process according to claim 1 in which the steel sheet is dipped inthe suspension of the insoluble phosphate during the pretreatingprocess.

5. A process according to claim 4 in which the temperature of the liquidsuspension is from 10 to 90 C.

6. A process according to claim 5 in which the steel sheets, after thepretreatment process are then squeezed with a roll.

7. A process according to claim 1 in which said insoluble phosphate iszinc phosphate.

8. A process according to claim 1 in which the insoluble phosphate iscalcium phosphate.

9. A process according to claim 1 in which the insoluble phosphate ismagnesium phosphate.

10. A process according to claim 1 in which the insoluble phosphate isferrous phosphate.

11. A process according to claim 1 in which the insoluble phosphate isferric phosphate.

12. A process according to claim 1 in which the insoluble phosphate isaluminum phosphate.

13. In a method of phosphatizing the surface of steel sheets or platedsteel sheets, wherein said method is preceded by a pret-reating process,the improvement according to which the pretreating process comprisesdipping said steel sheets in a liquid aqueous suspension, containing atleast 1 g./l. of zinc phosphate at a liquid temperature of 10 to 90 C.and at a pH of 3 to 10, then squeezing the sheets with a roll.

14. In a method of phosphatizing the surface of steel sheets or platedsteel sheets, wherein said method is preceded by a pretreating process,the improvement accord ing to which the pretreating process comprisesspraying 5 6 a liquid aqueous suspension, containing at least 1 g./l. of2,357,269 8/ 1944 Russell et al 1486.15 X zinc phosphate, at a liquidtemperature of 10 to 90 C., 2,479,423 8/1949 Snyder 148-6.15 onto thesurface of said steel sheets, said spraying pres- 2,501,846 3/1950Gilford 1486.15 sure being at least 0.5 kg./cm. and the pH of saidliquid 2,514,149 7/ 1950 Amundsen 148-6.15 bemg mm 3 5 FOREIGN PATENTSReferences Cited 21,870 10/ 1963 Japan,

UNITED STATES PATENTS ALFRED L. LEAVITT, Primary Examiner. 2,329,0659/1943 Lum et a1. 148-615 2,342,738 2/1944 Iernstedt 148 -6.16 10 BROWDYExammer'

